1. Field of the Invention
The present invention relates to the improvement of focus detection accuracy under light sources of different types in an auto-focus camera.
2. Description of the Related Art
Film/digital single lens reflex cameras commonly use a so-called TTL (Through The Lens) phase difference detection-type focus detection (hereinafter also referred to as AF) method in which a light flux passed through a photographing lens is split by a beam splitter, images of the respective light fluxes are formed on a focus detection sensor by two imaging lens with displaced optical axes, a defocus amount is calculated from a displacement between the two formed images, and focusing is achieved by driving the photographing lens according to the calculated defocus amount.
FIG. 14 illustrates an example of a phase difference detection-type focus detection apparatus that uses a secondary imaging system. In the apparatus, a field lens 102 that shares the same optical axis as a photographing lens 101 that performs focus detection is disposed in the vicinity of a predicted imaging plane F of the photographing lens 101. Two secondary imaging lenses 103a and 103b are placed side by side behind the photographing lens 101 and the field lens 102. Furthermore, light receiving sensors 104a and 104b are disposed behind the secondary imaging lenses 103a and 103b. The field lens 102 forms two different exit pupils 101a and 101b of the photographing lens 101 on pupil planes of the two secondary imaging lenses 103a and 103b. 
Consequently, light fluxes from an object surface S respectively entering the secondary imaging lens 103a and 103b become light fluxes output from equal-area regions of the exit pupils 101a and 101b of the photographing lens 101 which correspond to the respective secondary imaging lenses 103a and 103b and which do not overlap each other.
As described above, the received light amount distributions of the light fluxes from the equal-area exit pupils 101a and 101b on sensors 104a and 104b are ideally uniform. However, since a simple lens structure is required in consideration of a permissible size, cost, and convenience of production of the focus detection apparatus, lens aberration becomes relatively large. Consequently, the imaging relationship between pupils of the secondary imaging lenses 103a and 103b and the pupils 101a and 101b of the photographing lens 101 becomes incomplete, thereby causing a nonuniformity in the light amount or, in other words, causing shading such as that illustrated in FIG. 15 to remain on the sensors 104a and 104b. 
A focus detection apparatus arranged so as to remove a difference in light amount distribution between sensors 104a and 104b with respect to an object surface of uniform brightness to obtain the same light amount distribution is disclosed in Japanese Patent Application Laid-Open No. S60-101514. The invention described in the patent document compensates shading by assigning a weight coefficient to an amplitude of a photoelectric conversion output signal of a sensor according to positions of respective pixels on the sensor.
However, with the focus detection apparatus disclosed in Japanese Patent Application Laid-Open No. S60-101514 described above, depending on the type of light source illuminating an object, there may be cases where shading between sensors cannot be compensated.
When the focus detection apparatus disclosed in Japanese Patent Application Laid-Open No. S60-101514 is applied to a single lens reflex camera, a semi-transparent optical member 105 (main mirror) such as a beam splitter is disposed between the photographing lens 101 and the field lens 102 as illustrated in FIG. 16. The main mirror 105 is provided in order to split a light flux having passed through the photographing lens 101 at a predetermined ratio to a focus detection optical system and a finder optical system.
The characteristics of dependency on angle of incidence of a spectrum transmittance of the main mirror 105 are illustrated in FIG. 17. Approximately 40% of light with a wavelength of 600 nm or less is transmitted to the focus detection optical system. On the other hand, 40% or more of light of 600 nm or greater is transmitted to the focus detection optical system. The rate of transmittance gradually increases as the wavelength increases.
This is due to the spectrum transmittance of the main mirror 5 configured such that more near infrared light is transmitted. This characteristic is a result of a photoelectric conversion element as a auto-focusing sensor is sensitive to wavelengths up to around 1100 nm and performs focusing operations even at low brightness, and when a focusing operation cannot be performed under low brightness, a near-infrared (around 700 nm) light is irradiated by a light emitting diode from a camera to an object.
Meanwhile, a human eye is most sensitive to light ranging from around 450 to 650 nm. Light whose wavelength does not fall into this frequency range is not particularly important to a finder optical system from the perspective of color reproducibility.
Here, it should be noted that with respect to the optical configuration of the main mirror 105, the spectrum transmittance of the main mirror 105 is angle-dependent. In particular, with long-wavelength light of 600 nm or greater, transmittance varies according to the angle of incidence of a light beam.
The angles of incidence of light fluxes from the exit pupils 101a and 101b of the photographing lens 101 when being transmitted through the main mirror 105 differ from each other. Furthermore, the angles of incidence of light fluxes received at positions of respective pixels of the sensors 104a and 104b when being transmitted through the main mirror 105 also differ from each other. Therefore, shading of the sensors 104a and 104b varies depending on whether or not a light source irradiating the object includes a long-wavelength component.
FIG. 18 is a diagram illustrating spectral sensitivities of light sources, where the abscissa represents wavelength and the ordinate represents relative energy. In the diagram, a fluorescent light is denoted by F, a flood lamp is denoted by L, and a fill light described earlier is denoted by A.
The diagram illustrates that compared to components with longer wavelengths than 620 nm being almost absent among the wavelength components of the fluorescent light, with the flood lamp, the longer the wavelength, the stronger the relative sensitivity.
FIGS. 19A to 21B illustrate examples of shading waveforms and compensated waveforms of sensors in light sources of various types.
A shading waveform under a fluorescent light is illustrated in FIG. 19A. A result of compensation by performing an operation using an optimal compensation coefficient on the shading waveform of FIG. 19A is illustrated in FIG. 19B. In addition, a shading waveform under a flood lamp is illustrated in FIG. 20A. Since the angle of incidence of the light flux from the exit pupil 101a to the main mirror 105 is smaller than the angle of incidence of the light flux from the exit pupil 101b, the transmittance of near infrared light is high. Therefore, in FIG. 20A, the sensor 104a is shown obtaining a greater light amount than the sensor 104b. In addition, when taking cell positions of sensors into consideration, the further up on the sensor, the smaller the angle of incidence of a light flux to the main mirror. Therefore, the upper side of the sensor obtains a greater light amount than the lower side.
FIG. 20B illustrates a result of compensation performed on the shading waveform illustrated in FIG. 20A using the compensation coefficient computed from the shading waveform illustrated in FIG. 19A. As illustrated in FIG. 20A, uncompensated regions remain when the compensated waveform is not uniform.
In addition, a shading waveform under a fill light is illustrated in FIG. 21A. Since the fill light is near infrared light, the angular dependence of shading has increased as compared to FIG. 20A. FIG. 21B illustrates a result of compensation performed on the shading waveform illustrated in FIG. 21A using the compensation coefficient computed from the shading waveform illustrated in FIG. 19B. FIG. 21B shows that the remaining uncompensated region is even greater than in FIG. 20B.
As shown, depending on the type of light source irradiating an object, an uncompensated region of shading remains between sensors, causing a reduction in the detection accuracy of a displacement between two images.